What this MCCB carries — and what it means for your panel
The Siemens 3VA1225-5FF42-0BC0 is a 4-pole SENTRON molded case circuit breaker rated at a solid 125 A continuous, with a short-circuit interrupting capacity of 187 kA at 240 V — that's the kind of muscle you need for high-fault utility feeds or transformer secondaries where a standard 65 kA breaker would weld shut on a dead short. Line protection design means it's set up for feeder and main breaker duty, not motor branch circuit protection — no adjustable overload relays here, just fixed thermal-magnetic trip curves sized for cable and bus protection. The interrupting rating drops as voltage climbs: 121 kA at 415 V, 75.6 kA at 440 V, 30 kA at 500 V, and 17 kA at 690 V — so if your system runs 480 V or 600 V class, the 30 kA or 17 kA figure is the one your arc-flash study cares about.
Thermal derating — the number that changes your wire size
This breaker carries a full 250 A up to 50 °C ambient — plenty of headroom for a standard 125 A feeder. Above that, it starts to give: 243 A at 55 °C, 237 A at 60 °C, 230 A at 65 °C, and 223 A at 70 °C. If your panel sits near a hot process line or a bank of drives, that 70 °C figure means you're still at nearly double the 125 A rating, so no practical derating issue for the intended load.
Built-in auxiliaries and releases
Factory-fitted with two auxiliary switches (HQ type) for status feedback to a PLC or annunciator panel, plus an undervoltage release (UVR) that trips the breaker when control voltage drops — standard for safety circuits that need to drop a feeder on E-stop or power loss. Rated insulation voltage is 800 V, so it's comfortable on 480 V and 600 V class systems with margin. Power loss at full load is 59.5 W — factor that into your panel cooling calc if you're stacking several breakers in a closed enclosure.
Panel fit — dimensions and mounting
It's a 4-pole frame so the width is 140 mm — that's about 5.5 inches of DIN rail or panel-mount space. Depth is 70 mm, height 158 mm. Standard SENTRON 3VA footprint, so if you're replacing an older 3VL or 3VF series, check the hole pattern; the 3VA family uses the same mounting centres as the 3VL but verify against your existing backplate drillings.